1. Field of the Invention
The present invention relates generally to the field of medical devices for implantation into humans. More particularly, it concerns method for processing biological tissues for use as bioprosthetic devices.
2. Description of the Related Art
Bioprostheses are devices derived from processed biological tissues to be used for implantation into humans. The development of such devices originated as an attempt to circumvent some of the clinical complications associated with the early development of the mechanical heart valve, and has since resulted in a rapid proliferation of bioprosthetic devices for a variety of applications. Examples of some of the bioprostheses currently used or under development include heart valves, vascular grafts, biohybrid vascular grafts, ligament substitutes pericardial patches, etc.
The primary component of the biological tissues used to fabricate bioprostheses is collagen, a generic term for a family of related extracellular proteins. Collagen molecules consists of three chains of poly(amino acids) arranged in a trihelical configuration ending in non-helical carboxyl and amino termini. These collagen molecules assemble to form microfibrils, which in turn assemble into fibrils, resulting in collagen fibers. The amino acids which make up the collagen molecules contain side groups, including amine (NH2), acid (COOH) and hydroxyl (OH) groups, in addition to the amide bonds of the polymer backbone, all of which are sites for potential chemical reaction on these molecules.
Because collagenous tissues degrade very rapidly upon implantation, it is necessary to stabilize the tissue if it is to be used clinically. Chemical stabilization by tissue cross-linking, also referred to as tissue fixation, has been achieved using bi-functional and multi-functional molecules having reactive groups capable of forming irreversible and stable intramolecular and intermolecular chemical bonds with the reactive amino acid side groups present on the collagen molecules.
Glutaraldehyde is the most frequently used agent for cross-linking biological tissues. It is a five carbon aliphatic molecule with an aldehyde at each end of the chain, rendering it bifunctional. These aldehyde groups react under physiological conditions with primary amine groups on collagen molecules resulting in the cross-linking of collagen containing tissues.
Despite its widespread use, there are a number of drawbacks associated with glutaraldehyde cross-linking. For instance, under typical storage conditions, glutaraldehyde is self-reactive and will form a variety of polymeric and other species. As a result, a pure solution of monomeric glutaraldehyde becomes highly heterogeneous over time. The ratio of monomeric to polymeric species, the structure of the glutaraldehyde polymer, its formation kinetics, etc, have been described (for example, see Khor, 1997, and references cited therein).
The presence of polymeric glutaraldehyde species and the general heterogeneity of glutaraldehyde solutions can be problematic in a number of regards. For example, polymeric glutaraldehyde is less tissue permeable than low molecular weight forms. Thus, the use of glutaraldehyde solutions containing highly polymeric species can give rise to tissue that is not uniformly cross-linked, i.e. that contains regions of essentially native tissue within a cross-linked matrix. This non-uniformity can compromise the integrity/durability of the cross-linked tissue for many applications.
Another significant drawback associated with glutaraldehyde cross-linking is the propensity of the treated tissues to undergo calcification. Calcification appears to represent the predominant cause of failure of glutaraldehyde-fixed devices (Golomb et al., 1987; Levy et al., 1986; Thubrikar et al., 1983; Girardot et al., 1995). It is believed that the presence of polymeric forms of glutaraldehyde in the cross-linked tissue may contribute to such calcification, possibly by serving as a physical point of calcification (Thoma et al., 1987). In addition, the non-uniform cross-linking that can result from using heterogeneous glutaraldehyde solutions may also contribute to calcification because exposure of incompletely cross-linked regions following mechanical failure can result in calcification at the rapid rate typical of that for native, non-cross-linked tissue.
Yet another drawback to conventional glutaraldehyde cross-linking is that the polymeric product of glutaraldehyde can depolymize in vivo, causing the release of toxic monomeric glutaraldehyde. This leaching of glutaraldehyde can prevent the cellular growth on the bioprosthesis that is necessary for long term biocompatibility.
Thus, it is a significant disadvantage that polymeric forms of glutaraldehyde are present in the solutions generally used for cross-linking biological tissues. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above. In particular, a method has been developed in which tissue is fixed with glutaraldehyde, that is in a substantially monomeric form, thereby minimizing the complications associated with the use of heterogeneous glutaraldehyde solutions.